The manufacture of certain types of telephone cables require the shaping of a plastic coated steel strip about the cable core. This operation is advantageously carried out by continously running the sheathing device thus necessitating an availability of a continuous supply of sheathing material. For economical operation, it is manifest that the sheathing operation be not disrupted to effectuate the welding of a fresh supply of coated sheathing material to the trailing end of the about to be exhausted strip of sheathing material.
The dielectric plastic coating on the steel strip presents a problem in that it must be removed prior to the welding operations. Heretofore, one procedure that was followed included the steps of mechanically removing the plastic coatings from the trailing and leading ends of the new and old steel strips, and then placing the end sections of the respective strips in overlapping relation whereafter spot welds were made along the sections of the overlap. Subsequently, a substitute plastic coating must be pressure-temperature bonded to cover the large area of bare material. In this process, the trailing and leading ends of the strips are cut along diagonals so that the resultant welded joint is along a diagonal which facilitates the passage of the welded joint through the sheathing apparatus.
In U.S. Pat. No. 3,643,057 issued Feb. 15, 1972, to O. A. Becker, there is disclosed a method of welding two overlaying metal sheets coated with a thermoplastic material. The welding apparatus used to practice this method includes an auxiliary heating circuit which is connected to spaced sections of the metal sheets at locations where the plastic coating has been removed. The auxiliary circuit functions to resistance heat the intervening sections of metal to melt or destroy the plastic coating in the areas to be welded. Next, a pair of pressurized electrodes are moved into engagement with the melted coating to apply welding current to resistance weld the overlapping sections of the metal strips. In other instances, the patent suggests using a stream of hot air to melt the plastic coatings on the sheets prior to movement of the welding electrodes and the subsequent application of welding current to the joint.
In the use of alternating current spot welding machines, it is desirable to apply a series of welding pulses of the same magnitude during each welding cycle so as to obtain a neat, uniform line of weld spots having equal strength. Such welding apparatus often includes a first bus arm having an electrode mounted thereon and a second bus arm for supporting a second welding electrode. As the sheets to be welded are incrementally moved between said electrodes, and the electrodes are cyclically moved into engagement with the sheets, welding currents are applied through the respective bus arms to effectuate a series of spot welds. The current flowing through the bus arms sets up magnetic fields which are effective to induce eddy currents in the metal sheets, which together with hysteresis losses resulting from the magnetization and demagnetization of the sheet by the a.c. current cause an increase in impedance.
The cumulative effect of the eddy current and hysteresis losses and the consequent increase in impedance is to reduce the effective current delivered to the welding electrodes for the welding operations. In order to compensate for the increase in impedance during each subsequent welding cycle more and more current of greater magnitude must be supplied to the electrodes so that uniform welds of equal strength are produced.